1. Field of the Invention
The present invention relates to a transfer unit transferring an image on an image bearer onto a recoding medium, and an image forming apparatus such as a multifunctional apparatus equipped with at least one of copier, a printer, a facsimile and a plotter having the transfer unit.
2. Discussion of the Background
A transferer or a transfer unit used in an image forming apparatus is conventionally known to have a transfer roller and contact the transfer roller to a drum-shaped or an endless-belt-shaped image bearer upon application of pressure.
Such a transferee applies a bias voltage to the transfer roller or a roller facing the transfer roller, passes a transfer paper between the image bearer and the transfer roller to transfer a toner image formed on the image bearer onto a transfer paper with a pressure and an electrostatic force.
A typical second transfer unit 90 is shown in FIG. 15. A second transfer roller 5 is rotatably held by a transfer unit 91 and displaceably located in a direction contacting to and separating from an image bearer of an image forming apparatus (not shown) under biasing force of a spring 93.
The transfer unit 91 has a turning support point 94 for axially supporting the image forming apparatus or a both side unit (not shown) at the bottom. The second transfer unit 90 is pressurized by the both side unit (not shown) through a plate spring 95 when installed in the image forming apparatus so as to hit a protrusion 96 to a fixing unit frame (not shown) of the image forming apparatus such that the second transfer unit 90 is positioned therein.
The second transfer roller 5 contacts the image bearer of the image forming apparatus before the protrusion 96 and the spring 93 is compressed to obtain a predetermined transfer nip pressure.
Japanese published unexamined application No. 2007-148196 discloses an image forming apparatus, in which a second transfer unit is turnably held on the inside face of a both side unit and a L-shaped lock member projected from the inside face of a top edge thereof engages with a bar member of the image forming apparatus such that the second transfer unit and the both side unit are positioned therein, and both collars of the second transfer roller engage with a vertical positioning member of the image forming apparatus such that the second transfer roller is vertically positioned.
The second transfer unit is overall positioned in an image forming apparatus when protrusions 116 formed on inner both ends of the both side unit presses a contact part of 104 of the backside of the second transfer unit (the opposite side face of the second transfer roller).
Since the method of Japanese published unexamined application No. 2007-148196 is a press method with a protrusion as mentioned above, a large force is required to close the both side unit, resulting in deterioration of operability.
Namely, in either of the engagement between the lock member and the bar member or the contact between the contact part and the protrusion, the members serially operates each other, in other words, a positioning force operates in a linear direction and only a turning operation of the both side unit can position the second transfer unit. However, the operation force is inevitably large because of directly receiving a reaction force of the spring pressing the second transfer roller.
A force from the both side unit becomes large in proportion to a pressure of the second transfer roller to the image bearer, and a large force is required to close the both side unit in an image forming apparatus needing a large transfer pressure, resulting in deterioration of operability.
In addition, such a transferer presses the transfer roller to the image bearer even when a toner image is not transferred and the transfer roller is noticeably deformed with pressure if it is formed with a soft material such as sponge rubber and foamed urethane, resulting in defective transfer.
In order to solve this problem, the transfer roller is forcibly separated from the image bearer when a toner image is not transferred or a paper is jammed at a transfer site.
For example, when a predetermined time has passed since the final job was finished or a jamming occurred, a cam equipped in the image forming apparatus is driven to press the transfer roller or the transfer roller holding member such that the transfer roller and the image bearer are separated from each other.
When the both side unit and transfer unit are opened and closed while the transfer roller is separated from the image bearer, a force larger than a closing force while the transfer roller contacts the image bearer is required.
Namely, an extra force is required because the transfer roller and the image bearer are separated from each other by the cam.
Because of these reasons, a need exists for a transfer unit having improved operability and usability, capable of reducing a force for positioning the unit in an image forming apparatus.